SNAX/APN Application Programming Manual

SNAX/APN Application Programming Manual—420111-001
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The CRT Interface
This section describes the CRT interface, which supports block-mode communication
with display terminals and printers. Although the CRT interface is primarily used in
support of the Tandem Pathway transaction processing environment, it can be used by
any application process residing on a Tandem system.
The CRT interface includes support for:
•
Printers that use the SNA character string (SCS) and a defined set of SNA protocols
•
Display terminals
•
Printers that use the 3270 data stream and a defined set of SNA protocols
This section provides the following information:
•
Using the CRT protocol directly and indirectly
•
Sending and receiving 3270 data streams
•
A description of the change direction indicator (CDI) and the three different modes
associated with it
•
A list of functions available with the CRT protocol that you can use with your
applications.
Using the CRT Protocol Directly and Indirectly
The CRT interface for SNAX/APN enables an application process to send 3270 (or
SCS) data streams to a device and receive fields of information from it. The application
process maintains a full-screen buffer as the unit of transmission (rather than a line, for
example).
Applications can use the CRT interface to communicate directly with devices as though
the devices were ordinary files.
Applications can also use the CRT interface in conjunction with other Tandem software
to communicate indirectly with devices, as shown in Figure 2-1
. Although the figure
shows a terminal, the device could also be a printer.
The CRT interface serves a Pathway/TS terminal control process (TCP). The TCP
interprets a SCREEN COBOL requester program and uses the CRT protocol to
communicate with a specific SNA 3270 device. For information about the Pathway
environment and the SCREEN COBOL requester programs, see the Pathway/TS TCP
and Terminal Programming Guide and the Pathway SCREEN COBOL Reference
Manual.